Extracellular fluid (ECF) +} must be maintained within a narrow range. If ECF +] falls too low (hypokalemia), cell membranes hyperpolarize, and if ECF +] increases too much (hyperkalemia) cell membranes depobrize, both disrupt normal electrical excitability and can have life threatening cardiac effects. Kidneys and muscle work in concert to maintain ECF ]. During hypokalemia muscle ICF K is redistributed to buffer the fall in ECF }. During hyperkalemia K+ is pumped into muscle ICF until renal adjustments can occur. These important muscle specific homeostatic processes are only beginning to be understood at the molecular level. Evidence supports the hypothesis that K loss from muscle during hypokalemia results from decreased active K+ influx mediated by sodium pump (Na,KATPase, NKA) inhibition, and that K+ uptake during hyperktilemia is mediated by sodium pump activation. Our lab has established that during low K+ diet abundance of NKA subunits are depressed in an isoform and muscle specific manner: 60-95 percent fall in a2, not a 1. Using a novel K+ clamp technique, we recently showed that early in K+ restriction, prior to fall in a2, there is a severe blunting of both insulin stimulated K+ uptake, and of insulin stimulated redistribution of NKA ct2 type pumps from endosomes to the plasma membrane (PM). Evidence is mounting that the bumetanide sensitive Na,K,2C1 cotransporter also accounts for a component of muscle K+ influx and, thus, could play a role in potassium homeostasis. The overall aims are to determine the molecular mechanisms responsible for tapping muscle K+ stores during hypokalemia, for clearing excess plasma +] into the ICF store after K+ restoration, and to understand how these processes are altered in a set of clinically relevant paradigms. The contribution of both Na,K-ATPase isoforms and NKCCI in both red oxidative white glycolytic muscle will be studied with a compartmental analysis approach in which the following are assessed: whole body K+ uptake, muscle specific K+ transport, subcellular distribution and activity of K+ transporters, and pool size regulation of K transporter protein and mRNA levels. Aim 1 will test the hypothesis that the shift of K+ to ECF during K restriction is mediated by decreased plasma membrane (PM) expression of both NKA a2 and NKCC1 coupled to resistance to insulin stimulated K+ uptake, and that this process is altered in uremia accompanying chronic renal failure. Aim 2 will test the hypothesis that thyroid hormone or dexamethasone, both of which increase NKA cx2 (and perhaps NKCC 1), alter extrarenal control of K+ horneostasis. Aim 3 will test the hypothesis that the uptake of K+ from ECF to ICF during K+ restoration (following K+ restriction) is mediated by normalizing surface expression of both NKA a2 and NKCC1. Accomplishing these aims will identify the cellular mechanisms responsible for tapping and repleting the muscle K+ reservoir, which will, ideally, suggest strategies to manipulate muscle K stores in clinical settings.